1. Field of the Invention
This invention relates to electronic ballasts for discharge lamps such as fluorescent lamps and, in particular, to such ballasts which are useful in applications where compact size is desirable.
2. Description of Related Art
There is an ever-increasing demand for smaller electronic ballasts. In order to achieve this goal, a variety of problems must be overcome, for example:
If the operating frequency of ballast circuitry is increased sufficiently to achieve a substantial reduction in the sizes of magnetic and filtering elements, power losses tend to increase.
Decreasing the size of filtering components tends to have a negative impact on circuit performance.
It is an object of the invention to provide an electronic ballast for discharge lamps which can be made compact without substantially increasing cost or adversely affecting performance.
In accordance with the invention, an electronic ballast includes a source of power having a varying DC voltage, a self-oscillating converter powered by the DC voltage for producing pulses at a controlled frequency, a resonant tank for converting the pulses to an alternating current for powering a discharge lamp, and a feed-forward control circuit for effecting variation of the frequency in response to variations of the DC voltage, the resonant tank having a voltage drop which varies directly with said frequency.
Such a ballast does not require a regulated DC power supply and will operate effectively with a filter capacitor that is much reduced in size from capacitors that are typically used to reduce ripple of the rectified DC power.
A self-oscillating resonant converter circuit for operating discharge lamps, which does not require a regulated DC power supply, is described U.S. Pat. No. 6,072,710. However, that circuit utilizes a feed-back approach. In accordance with the invention, a feed-forward approach is utilized which is inherently more stable and has a greater range of correction for variations of the DC source voltage.